Problem with Startup Security Utility to allowing booting from external media.

I have a new MacBook Pro (16-inch, 2019), and am trying to use the Startup Security Utility to allow booting from external media, my backups made with SuperDuper.

However, when I try to permit booting from external media with the Startup Security Utility, it refuses to accept my administrator password. On the other hand, if I boot normally, I can log in with the administrator username and password with no difficulty. The problem is that the Startup Security Utility does not accept this password. I have tried several times (in case of typos), but there is always the same problem.

How do I get the Startup Security Utility to recognize my valid administrator password?

Note that I have no firmware password.

Thanks in advance.